Tidal Sand Ridges Research Articles

Transgressive-barrier and shallow-shelf interpretation of the lower Paleozoic Peninsula Formation, South Africa

The Peninsula Formation of the lower Paleozoic Cape Basin is well exposed on the Cape Peninsula, where it comprises a 750-m-thick quartz arenite complex transgressively overlying back-barrier tidal flat deposits. Five sandstone facies are distinguished on the basis of distinct assemblages of sedimentary structures. The two lowermost facies are relatively thin and consist of northwestward (landward)-dipping plane-bedded sandstones, interpreted as barrier washover deposits, and small-scale back-barrier tidal channels, with cross-bedding of variable direction. At a higher stratigraphic level are large-scale channel deposits, which are ascribed to the lateral migration of tidal inlets. These sequences are characterized by ebb-oriented cross-bedding at the base, with smaller scale sets of longshore-directed or flood-oriented cross-bedding in the upper parts. A facies interpreted as beach foreshore, comprising seaward-dipping plane beds with multiple discordances and heavy mineral laminae, is occasionally preserved above the inlet sequences. The upper part of the Peninsula succession is dominated by tabular and lenticular sandstone bodies with megasets as much as 10 m thick showing complex internal organization of smaller structures. These intrasets are mainly trough crossbeds that are inclined either in the same direction as the major foresets or at a variable angle to them. This facies is compared with the tidal sand ridges and sand waves of North Sea type. A large proportion of the Peninsula tidal sand bodies migrated southwestward parallel to the shoreline; others are directed offshore. The considerable thickness of the Peninsula transgressive sandstone is attributed to gradual subsidence balanced by substantial sediment supply, introduced mainly by a braided alluvial complex to the northwest. This led to vertical stacking of facies. Barrier inlet migration was rapid compared with the rate of transgression, thereby obliterating many of the features characteristic of barrier island deposits. The several-hundred-metre thickness of tidal sand body deposits indicates that the water depth remained within the critical limits of tidal current dominance. This regime was eventually terminated by marine regression and the accumulation of fossiliferous lagoonal siltstones and glaciogenic diamictites. It is suggested that deposition of the Peninsula Formation was strongly influenced by storm processes. Extensive barrier washover sheet sands are attributed to storm surge. Furthermore, the two-part sequence within the tidal sandbar facies of a basal pebbly unit overlain by plane-bedded, fine-grained sandstone without heavy mineral segregation is identical to modern storm-generated sequences off Long Island, New York. The perpendicular to oblique relationship of shoreface channels to the tidal sandbars suggests incision by storm surge ebb currents, which deposited a pebbly lag on the bar toes.

Tidal sand ridges and shoal-retreat massifs

In 1963, Off defined a bedform type which he described as rhythmic linear sand bodies caused by tidal currents. He figured twelve examples from around the world. Since then, the morphology and dynamics of sand transport in one of these areas, the tidal shelf seas around Great Britain, have undergone intensive study. The tidal sand ridges emerge as anomalies, in that they do not fit into the sequence of morphologic provinces which characterize the major sediment transport paths. It is suggested here that the ridge fields are analogous to the shoal-retreat massifs of the Middle Atlantic Bight in that they have been inherited from a nearshore regime during the course of the Holocene transgression. Shoal-retreat massifs are low, broad, shelf-transverse sand bodies which mark the retreat paths of coastal depocenters associated with littoral drift convergences. Two main types of shoal-retreat massifs in the Middle Atlantic Bight are: (1) estuarine shoal-retreat massifs; and (2) cape shoal-retreat massifs. Two similar classes of shoal-retreat massifs may develop in tidal shelf seas, but the mechanism is somewhat different. Class-1 tidal massifs are tidal ridge fields whose ridges were hydraulically packaged in an estuarine environment. If, upon transgression, they find themselves in a broad tidal bight which continues to funnel tidal flow, the ridges may survive for long distances out into the bight. The ridge fields of the Southern Bight of the North Sea may have undergone such an evolution. Class-2 tidal massifs occur off promontories in tidal seas that are swept by the edge waves generated by amphidromic tidal systems. Here the debris of shoreface erosion tends to be stored as shoreface-connected, tide-maintained ridges. Such ridges are also pre-adapted to survive with modification for long distances out on the associated shelf, as the water column deepens during a marine transgression.